RO and UF membranes for the separation of a liquid from a second component, such as another liquid or dissolved or suspended solids, have been known for many years, and a variety of semipermeable membranes have been developed for use in such RO and UF processes. More recently, semipermeable NF membranes have been developed, and membranes useful in these three processes are generally referred to hereinafter as cross flow (CF) membranes which is meant to indicate that they are intended for use in cross flow filtration processes, e.g. spiral wound cartridges and the like wherein the flow is past a semipermeable membrane surface and a concentrate stream is withdrawn from an opposite end of the feed flow path. These membranes, while satisfactory in achieving such separations, must be incorporated into a cartridge or filter module of some sort to provide a practical realization of the potential membrane performance. Several shortcomings of conventional CF filtration modules have been noted, some of which result from particular combinations of the semipermeable membrane and other material layers that are adjacent to, and often in contact with, the membrane to provide the needed liquid flow patterns.
As an example, RO cartridges of the spiral wound type have employed various materials as backing and permeate carriers for separately cast semipermeable membranes, which combinations were commonly wound together with brine-side spacers to create a spiral cross flow assembly. One type of backing material advocated for use in the early days of cartridge development comprised particles of an incompressible substance, such as glass beads, attached to a porous substrate, as described in U.S. Pat. No. 3,367,505. A more common type of backing material that has been used for many years as a permeate carrier is an epoxy-impregnated tricot which is often sandwiched as a single layer permeate carrier between two sheets of separately produced semipermeable membrane that have been cast on appropriate backing layers and then joined at their edges to create an envelope about the permeate carrier.
Permeate carriers withdraw the permeate from the low-pressure side of the semipermeable membrane and carry it to an exit. In general, two areas where improvements have been sought in such cross flow filtration systems are (a) in the throughput, or rate of collection, of permeate and (b) in the maintenance interval or service life of the filtration equipment; these two areas are related to each other in certain aspects. For example, the throughput or collection rate of permeate is directly related to the pressure applied across the semipermeable membrane; in general, the higher the applied pressure, the greater the rate at which permeate flows to the low pressure side of the membrane. However, higher pressures require higher amounts of energy, and applied pressures cannot be arbitrarily increased without limit because most semipermeable membranes do not have a physical structure capable of withstanding very high pressures. Moreover, the application of excessive pressure to certain semipermeable membranes may cause rupture of the membrane. When elevated pressures are employed not only to force the permeate through the membrane, i.e. as by overcoming the osmotic pressure of the solution, but also to drive the permeate along its flow path to a collection point in a spiral wound cartridge, the pressure losses that are experienced are sometimes referred to as side pressure loss, i.e., the resistance to flow of the permeate along or through a permeate carrier. The efficiency of a substrate material to permit liquid flow in the plane thereof has more recently been measured on a numerical scale to produce what is termed an H value, as described hereinafter.
In addition to providing physical support for the separately cast semipermeable membrane so as to prevent its stretching, sagging or rupture, such backing material has always functioned in a typical CF filtration module to provide an adequate flow path for the permeate to a collection point, i.e. as a permeate carrier. For example, typical CF filtration systems in widespread use today use a three-layer arrangement, including a semipermeable membrane layer, a mechanical reinforcing or anti-bagging support layer upon which the semipermeable membrane is directly cast, and a permeate carrier layer that primarily provides a flow path for the permeate. A popular permeate carrier in use today is comprised of tricot material, which is a knitted fabric, e.g. polyester, that is generally epoxy or melamine-coated.
A manufacturing process for making such spirally-wound semipermeable membrane cartridges for cross flow filtration purposes is illustrated in U.S. Pat. No. 4,842,736. Very generally, it has been traditional that a sheet of permeate carrier material 12 is wound about a porous collection tube 18 and additional leaves are joined thereto. In the arrangement, one such leaf of permeate material separates one folded sheet from the adjacent folded sheet of semipermeable membrane; sheets of feed supply material 16 are placed between the membrane surfaces of the facing halves of the folded membranes. These membrane sheets are formed by casting a base layer of semipermeable membrane 46 upon a porous felt backing layer 48, leaving a free upper surface upon which a thin, more discriminating layer can be formed for RO or NF purposes. In such a spirally-wound cartridge, the brine or other impure liquid being treated in the separation process is traditionally supplied to the active surfaces of the membrane by pumping it in an axial direction through the feed supply sheets disposed between the opposing faces of the folded membrane. This has proved to be a construction that is in wide use throughout the United States for NF and RO membranes, which membranes are commonly made from polyamide through an interfacial reaction upon the surface of a polysulfone base membrane (which may itself have UF capability) that had previously been cast upon a thin felt support layer. U.S. Pat. No. 4,855,058 shows a generally similar construction where permeate is discharged axially from one end of the filtration element; however, difficulty with seals has prevented widespread commercial acceptance.
U.S. Pat. No. 3,813,334 shows a spiral wound semipermeable membrane cartridge designed for cross flow filtration wherein the semipermeable membrane is cast in sheet form and then associated, as a part of a spirally-wound cartridge, with a first layer of felt, such as Dacron polyester felt sold commercially as Reemay 0601 that lies adjacent a resin-impregnated Dacron polyester tricot. The tricot fabric is specially formed so as to have an array of parallel wide ribs and narrow grooves that provide alternating channels in its distal surface, which abuts a similar surface. The cartridge thus includes a plurality of envelopes of membrane, bonded one to another along three edges, with such two-ply backing material facing each other and staggered with the ribs aligned with the opposite grooves or channels that provide open flow passageways through which the permeating liquid can readily flow to a central collection tube.
U.S. Pat. No. 5,500,167 teaches making a filtration medium for use in treatment filters wherein a porous nonwoven fibrous support material is coated with a first casting solution having properties so as to produce pores of a relatively large size, which is in turn coated with a second casting solution designed to create a porous material having pores of a substantially smaller size. The casting solutions are of polymeric material, such as polysulfone, and are simultaneously gelled so as to create an integral structure incorporating the nonwoven substrate. U.S. Pat. No. 5,804,280 teaches making a composite filtration material wherein a polysulfone disk having concentric grooves on opposite surfaces that drain to a central channel is covered on each surface by a wet laid, polyester, nonwoven fibrous felt that serves as a support and drainage medium; it is in turn covered by a polyamide fiber porous medium and then by layers of spunbond polypropylene. The overall arrangement is clamped between aluminum plates and then subjected to solvent bonding to join the adjacent layer to the substrate to result in a composite structure.
Although tricot material has frequently been a popular choice as a permeate carrier, other carrier materials have been investigated in a search to find even better materials, e.g., having even lower resistance to liquid flow; for example, non-woven polypropylene carriers have been used in a few instances. U.S. Pat. No. 4,802,982 discloses spiral wound membrane constructions for use in UF and RO separation processes wherein semipermeable membranes are cast upon a polypropylene or polyester felt backing layer and then associated with a ribbed permeate carrier. Some polytetrafluoroethylene membranes were laminated onto a polyester tricot carrier to provide cross flow constructions that are then compared to the inventive arrangement wherein the permeate pathway within each semipermeable membrane envelope constitutes a pair of carrier layers that employ parallel, cylindrical ribs interconnected by a web-like matrix, which ribs extend in the direction of permeate flow.
Although such specialty arrangements have proved useful in some limited areas, commercial manufacturers of cross filtration cartridges have generally continued to rely on tricot knitted material to provide the permeate carrier layer. One combination of materials frequently used in UF cross flow filtration cartridges includes a polysulfone membrane, typically about 4 mils in thickness, that has been cast on a felt backing material about 3–5 mils in thickness; such is associated with a tricot permeate carrier, such as K-1015 Hornwood tricot having a 48 wale rating, i.e., 48 threads per inch, and rolled to make cartridges from such materials, using a fabrication method such as that generally disclosed in the '736 patent.
More advantageous arrangements for CF membrane cartridges continue to be sought, particularly ones which will permit economies of production and thus provide commercial advantages.